I'm not sure whether to expect a metric gigabit (1000 metric MBit) or a binary one (1024 binary MBit), but looking at our results our best is 936 Mbit/s and worst 308 Mbit/s. All of them should thus ideally reach at least 936 MBit/s. They all have gigabit network card.

And now, try to improve it:
I went through the whole shebang with
sudo ifconfig eth1 mtu 9000
sudo ifconfig eth1 mtu 8000
etc.
Anyway, I got the following MTUs that way:
Be 7100
B 7100
Ne 9000
Ta 9000

I then set the MTUs to 7100 on all the nodes and tried pinging from node to node, e.g.:
ping -s 7072 -M do 192.168.1.101

Well, that maxed out at 1472 i.e. about MTU 1500 which was the original value. So I'm a bit confused.